Constructors Destructors Review What is a constructor It

Constructors & Destructors Review

What is a constructor? • It is a member function which initializes a class. • A constructor has: (i) the same name as the class itself (ii) no return type

class rectangle { private: float height; float width; int xpos; int ypos; public: rectangle(float, float); // constructor void draw(); // draw member function void posn(int, int); // position member function void move(int, int); // move member function }; rectangle: : rectangle(float h, float w) { height = h; width = w; xpos = 0; ypos = 0; }

Comments on constructors • A constructor is called automatically whenever a new instance of a class is created. • You must supply the arguments to the constructor when a new instance is created. • If you do not specify a constructor, the compiler generates a default constructor for you (expects no parameters and has an empty body).

Comments on constructors (cont. ) void main() { rectangle rc(3. 0, 2. 0); rc. posn(100, 100); rc. draw(); rc. move(50, 50); rc. draw(); } • Warning: attempting to initialize a data member of a class explicitly in the class definition is a syntax error.

Overloading constructors • You can have more than one constructor in a class, as long as each has a different list of arguments. class rectangle { private: float height; float width; int xpos; int ypos; public: rectangle(float, float); // constructor rectangle(); // another constructor void draw(); // draw member function void posn(int, int); // position member function void move(int, int); // move member function };

Overloading constructors (cont. ) rectangle: : rectangle() { height = 10; width = 10; xpos = 0; ypos = 0; } void main() { rectangle rc 1(3. 0, 2. 0); rectangle rc 2(); rc 1. draw(); rc 2. draw(); }

Composition: objects as members of classes • A class may have objects of other classes as members. class properties { private: int color; int line; public: properties(int, int); // constructor }; properties: : properties(int c, int l) { color = c; line = l; }

Composition: objects as members of classes (cont. ) class rectangle { private: float height; float width; int xpos; int ypos; properties pr; // another object public: rectangle(float, int, int ); // constructor void draw(); // draw member function void posn(int, int); // position member function void move(int, int); // move member function };

Composition: objects as members of classes (cont. ) rectangle: : rectangle(float h, float w, int c, int l): pr(c, l) { height = h; width = w; xpos = 0; ypos = 0; }; void main() { rectangle rc(3. 0, 2. 0, 1, 3); C++ statements; }

What is a destructor? • It is a member function which deletes an object. • A destructor function is called automatically when the object goes out of scope: (1) the function ends (2) the program ends (3) a block containing temporary variables ends (4) a delete operator is called • A destructor has: (i) the same name as the class but is preceded by a tilde (~) (ii) no arguments and return no values

class string { private: char *s; int size; public: string(char *); // constructor ~string(); // destructor }; string: : string(char *c) { size = strlen(c); s = new char[size+1]; strcpy(s, c); } string: : ~string() { delete []s; }

Comments on destructors • If you do not specify a destructor, the compiler generates a default destructor for you. • When a class contains a pointer to memory you allocate, it is your responsibility to release the memory before the class instance is destroyed.

What is a copy constructor? • It is a member function which initializes an object using another object of the same class. • A copy constructor has the following general function prototype: class_name (const class_name&);

class rectangle { private: float height; float width; int xpos; int ypos; public: rectangle(float, float); // constructor rectangle(const rectangle&); // copy constructor void draw(); // draw member function void posn(int, int); // position member function void move(int, int); // move member function };

rectangle: : rectangle(const rectangle& old_rc) { height = old_rc. height; width = old_rc. width; xpos = old_rc. xpos; ypos = old_rc. ypos; } void main() { rectangle rc 1(3. 0, 2. 0); // use constructor rectangle rc 2(rc 1); // use copy constructor rectangle rc 3 = rc 1; // alternative syntax for // copy constructor C++ statements; }

Defining copy constructors is very important • In the absence of a copy constructor, the C++ compiler builds a default copy constructor for each class which is doing a memberwise copy between objects. • Default copy constructors work fine unless the class contains pointer data members. . . why? ? ?

#include <iostream. h> #include <string. h> class string { private: char *s; int size; public: string(char *); // constructor ~string(); // destructor void print(); void copy(char *); }; void string: : print() { cout << s << endl; }

void string: : copy(char *c) { strcpy(s, c); } void main() { string str 1("George"); string str 2 = str 1; // default copy constructor str 1. print(); // what is printed ? str 2. print(); str 2. copy("Mary"); str 1. print(); // what is printed now ? str 2. print(); }

Defining a copy constructor for the above example: class string { private: char *s; int size; public: string(char *); // constructor ~string(); // destructor string(const string&); // copy constructor void print(); void copy(char *); };

string: : string(const string& old_str) { size = old_str. size; s = new char[size+1]; strcpy(s, old_str. s); } void main() { string str 1("George"); string str 2 = str 1; str 1. print(); // what is printed ? str 2. print(); str 2. copy("Mary"); str 1. print(); // what is printed now ? str 2. print(); } Note: same results can be obtained by overloading the assignment operator.